Hiroshi Kobayashi

To estimate the polyamine distribution in bovine lymphocytes and rat liver, the binding constants (K) for DNA, RNA, phospholipid, and ATP were determined under the conditions of 10 mM Tris-HCl, pH 7.5, 2 mM Mg2+, and 150 mM K+. The binding constants of spermine for calf thymus DNA, Escherichia coli 16 S rRNA, phospholipid in rat liver microsomes and ATP were 1.15 x 10(2), 6.69 x 10(2), 2.22 x 10(2), and 5.95 x 10(2) M-1, respectively. From these binding constants and experimentally determined cellular concentrations of macromolecules, ATP, and polyamines, spermine distribution in the cells was estimated. In bovine lymphocytes, the mols of spermine bound to DNA, RNA, phospholipid, and ATP were 0.79, 3.7, 0.23, and 4.3 per 100 mol of phosphate of macromolecules or ATP, respectively. In rat liver, they were 0.19, 1.0, 0.05, and 0.97/100 mol of phosphate of macromolecules or ATP, respectively. The binding constants of spermidine for macromolecules and ATP were smaller than those of spermine, but a similar tendency was observed with spermidine distribution among macromolecules and ATP in the above two cells. The amount of polyamine bound to DNA and phospholipid was significantly lower than that to RNA. When either the Mg2+ or K+ concentration increased, the amount of free spermine and that bound to RNA and ATP increased, but the amount of spermine bound to DNA and phospholipid decreased. The results indicate that most polyamines exist as a polyamine-RNA complex in cells. Under the conditions that globin synthesis is stimulated by spermine in a rabbit reticulocyte cell-free system, the amount of spermine bound to RNA was very close to the value estimated in the cells.

Action of purified human cathepsin B on recombinant single-chain urokinase-type plasminogen activator (pro-uPA) generated enzymatically active two-chain uPA (HMW-uPA), which was indistinguishable by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot from plasmin-generated HMW-uPA and from elastase- or thrombin-generated inactive two-chain urokinase-type plasminogen activator. Preincubation of cathepsin B with E-64 (transepoxysuccinyl-L-leucylamino- (4-guanidino)butane, a potent inhibitor for cathepsin B) prior to the addition of pro-uPA prevented the activation of pro-uPA. The cleavage site within the cathepsin B-treated urokinase-type plasminogen activator (uPA) molecule, determined by N-terminal amino acid sequence analysis, is located between Lys158 and Ile159. Pro-uPA is cleaved by cathepsin B at the same peptide bond that is cleaved by plasmin or kallikrein. Binding of cathepsin B-activated pro-uPA to the uPA receptor on U937 cells did not differ from that of enzymatically inactive pro-uPA, indicating an intact receptor-binding region within the growth factor-like domain of the cathepsin B-treated uPA molecule. Not only soluble but also tumor cell receptor-bound pro-uPA could be efficiently cleaved by cathepsin B to generate enzymatically active two-chain uPA. Thus, cathepsin B can substitute for plasmin in the proteolytic activation of pro-uPA to enzymatically active HMW-uPA. In contrast, no significant activation of pro-uPA by cathepsin D was observed. As tumor cells may produce both pro-uPA and cathepsin B, implications for the activation of tumor cell-derived pro-uPA by cellular proteases may be considered.